1. Field of the Invention
The present invention relates to a supercapacitor and a method for preparing the same, and more particularly, to a supercapacitor having a metal oxide electrode coated onto titanium dioxide ultrafine fiber and a method for preparing the same.
2. Description of the Background Art
Generally, a super capacitor is largely divided into an active carbon-based capacitor, a metal oxide-based electrochemical pseudo capacitor, and an oxidizable/deoxidizable conductive high polymer-based capacitor according to an electrode material.
The active carbon-based capacitor is based on the principle that ions of an electrolyte solution form an electric double layer capacitor (EDLC) on a surface of an electrode and are physically absorbed/detached to/from the EDLC. In the active carbon-based capacitor, carbon is chemically stable, and thus, the lifespan of the active carbon-based capacitor is excellent. However, since charges are accumulated only on the EDLC of the electrode surface, capacitance of the active carbon-based capacitor is lower than that of a supercapacitor using the Faraday reaction.
The metal oxide-based pseudo capacitor is a supercapacitor using a metal oxide as an electrode active material. The electrode active material of the metal oxide-based pseudo capacitor requires a high conductivity. When the metal oxide-based pseudo capacitor has a low conductivity, an equivalent series resistance (ESR) thereof is increased and thus the output efficiency thereof is lowered. When the substantial utilization degree of the active material is lowered, the energy density is lowered. That is, an electrode formed of the particle active materials has an increased resistance between the particles, which reduces function, resulting in a low power density per unit weight. A carbon material may be mixed to the metal oxide capacitor in order to increase a conductivity. The carbon material h as to be serially connected to t he active material of the electrode at a current collector inside the electrode in order to obtain an excellent efficiency. Accordingly, degree of dispersion of the carbon material and a mixing method between the active material and the carbon are very important. Substantially, it is difficult to maintain an excellent degree of dispersion of the carbon material.
In a conventional metal oxide capacitor, an electrode is fabricated by coating a metal oxide with a conductive high polymer. Also, an electric double layer capacitor (EDLC) using a carbon fiber has been proposed, in which a mechanical intensity of the electrode is high, several small capacitors can be fabricated, capacitance per unit volume is high, and current leakage is relatively low.
A supercapacitor using ruthenium oxide has a current accumulation mechanism in which protons move while metal oxides are oxidized or deoxidized by the Faraday reaction. Therefore, the supercapacitor using ruthenium oxide has a higher capacitance than the EDLC using carbon. However, the ruthenium oxide is several hundred times more expensive than a general active material. Therefore, it is difficult to utilize the ruthenium oxide as an electrode active material. In order to solve the problem, a technique for fabricating an electrode using a minimum amount of ruthenium oxide is required.
Ruthenium oxides used in the conventional supercapacitor are provided in a sol-gel state and have a weak bonding force between particles. Thus, in order to apply ruthenium oxide to an electrode, a bonding material, which increases an electrode resistance and thus decreases capacitance of the capacitor, is required. In order to solve this problem, a method for fabricating an electrode by immersing a titanium dioxide-based substrate or a carbon-based substrate into ruthenium chloride solution or ruthenium oxide solution and then thermally processing the substrate has been in research. However, this method has a disadvantage in that a wide specific surface area of the substrate cannot be utilized since the ruthenium oxides are not evenly distributed on the substrate.
In order to achieve metal oxide supercapacitor having a high capacitance, the contact resistance between active materials has to be increased and the surface area of expensive metal oxides has to be increased such that a minimum amount of the metal oxides is required.